\begin{table}%t3
 \caption{Summary of data remaining after pre-selection cuts.}
  \label{tab:selected_data}
\setlength{\tabcolsep}{10pt}
\renewcommand{\arraystretch}{1.1}
%%\centering
\par
     \begin{tabular}{llll}	
        \hline	
        \hline	
        Observation period & $T_{{\rm on}}$(min) 
	& $T_{{\rm off}}$(min)& $T_{{\rm on}}$/$T_{{\rm off}}$\\
        \hline
        3 Oct.--18 Nov. & 1301 & 969 &1.34\\
        2000&&&\\
        20 Sep.--15 Nov. & 1658 & 1448 &1.15\\
        2001&&&\\
        \hline
        Total & 2959 & 2417 & 1.22\\
        \hline
      \end{tabular}
 \end{table}